Can anyone identify this tree for me and explain what the large green leaves are doing on the same tree?

Looks like Acer Flamingo, I think it's called, though I can't remember it's full name. It's a hybrid, and hybrids have a tendency to revert; cut off the rogue leaves.

Or it has been grafted and the large green leaves are coming up from the root stock. Don't 'cut' the rogue leaves but use your thumb to 'push' them off at the stem.

Bulbaholic's right, if the growth is coming from the bottom of the plant. I thought it was growing from further up from the picture, but it is hard to tell.

Brilliant thanks guys for the quick response. The green leaves are quite high up so I'll cut them off.